1. Field of the Invention
The invention generally relates to scanning tunneling measuring devices, and more particularly to a scanning tunneling microscope probe used for scanning a sample surface.
2. Description of the Related Art
The scanning tunneling microscope (STM) has become one of the most powerful tools used in studying the surface structure of electrically conducting solid state materials at an atomic resolution. Descriptions of the general structure and operation of STMs are generally provided in U.S. Pat. No. 4,343,993 issued to Binnig et al.; Binnig, G. et al., “Surface Studies by Scanning Tunneling Microscopy,” Phys. Rev. Lett., Vol. 49, No. 1, pp. 57-61, Jul. 5, 1982; and Binnig, G. et al., “Scanning Tunneling Microscopy—from Birth to Adolescence,” Rev. Mod. Phys., Vol. 59, No. 3, Part. 1, pp. 615-625, July 1987, the complete disclosures of which, in their entireties, are herein incorporated by reference.
By raster-scanning a single sharp probing tip above a sample surface within a vertical distance of a few nanometers, the topological structure of the sample surface can be mapped out at the resolution of an atomic scale. This topological detail is detected through the intensity of the electron tunneling current between the probing tip and the surface atomic protrusions. An electrical bias of a few volts (for example, approximately 1 to 5 volts) is applied between the tip and sample. Moreover, a piezo drive tube is used to control the position and movement of the probing tip.
Since its conception, the STM has had the greatest impact in the field of modern surface science because of its superior capability of characterizing and resolving the surface atomic structures and defects. Surface features such as atomic point defects, dislocations, and grain boundary identification can routinely be studied using a STM. Furthermore, STMs also allow the characterization of step structures at the atomic level during the processes of surface preparation and growth of semiconductors, such as epitaxial growth on semiconductor structures.
However, even though the atomic resolution of a surface can routinely be obtained using a STM, the interpretation of the experimental results is often quite difficult, or even impossible due to the following reasons. For example, it is well known that maxima in topographic STM images may not necessarily correspond to atomic positions. This is particularly the case for semiconductor surfaces in which the effects of surface electronic structure play an important role for the interpretation. In the case of metals, effects of electronic structure play only a minor role. However, an atomically-resolved disordered surface structure cannot be interpreted in a unique way. Moreover, interference of electronic states from several different atomic sites, and possible image contrast reversal as a function of tip-sample separation due to the interaction between the probe tip and the sample surface, may prevent the extraction of reliable information from STM images of disordered sample surfaces.
Unfortunately, STMs have been generally unable to resolve amorphous structures atomically. For quasi-crystalline structures, STM studies have revealed lattices with atomic resolution. However, it is extremely difficult, and in some cases practically impossible, to extract the atomic information from experimental results because the assignment of maxima in topographic STM images to particular atomic species has generally been unclear and unfocused. As such, the industry has suggested that at least some imaged reference lattice is required simultaneously to facilitate the interpretation of experimental results.
FIG. 1 illustrates the physical structure of a conventional single-tip STM system and a sample surface on which it is probing. During operation, the single-tip microscope probe 2, which in the conventional systems, generally comprises a single probing tip 25, which moves across the sample surface 1 with its motions controlled by a piezo controller 3 in nanometer precision. An electrical bias 5 is applied between the probing tip 25 and the surface atoms 9 on the sample surface 1. An electronic current (A) feedback monitoring circuit 6, which is controlled by an electronic control circuit 4, detects the interaction between the probing tip 25 and surface atoms 9. The output of the electronic current feedback monitoring circuit 6 directly reflects the interaction of the probing tip 25 and the sample surface 1.
Further processing of the electronic signals provides the topological structure of surface atoms 9. FIG. 2 shows such an interactive mechanism of a conventional single probe STM illustrating electron wave functions 7 of the surface atoms 9 of the sample surface 1 and the electron wave function 8 of the protruding atom 10 of the probing tip 25. The double arrows in FIGS. 1 and 2 indicate the horizontal bi-directional scanning movement of the probing tip 25.
Generally, conventional STMs are not primarily sensitive to atomic positions, but rather to those electronic states that protrude into the vacuum region (the entire system is contained inside a vacuum chamber, which is not shown in the figures) and significantly overlap with the electron wave function 8 of the probing tip 25. The information obtained reflects a convolution of electronic states at the sample surface 1 and the apex (protruding atom 10) of the probing tip 25. Thus, in conventional STMs, it is desirable to use tips with a less prominent electronic structure (for example, the ground state of a metal atom) in order to exclude the influence of the tip's electronic structure.
Additionally, many other types of probing microscopes, such as atomic force microscopes (AFM), magnetic force microscopes (MFM), and optical tunneling microscopes (OTM) have also been developed in the industry. Generally, these other types of probing microscopes are based on the interaction between single probing tips and the sample surface. Moreover, these other types of probing microscopes also generally face the same challenge of the interpretation of results as found in conventional STMs. Therefore, there remains a need for a novel probing microscope capable of facilitating the interpretation of experimental results of sample surface probing.